The biosynthesis of chlorophyll has been demonstrated to require an extensive set of enzymes, the initial stages of which are shared with the synthesis of heme. Of these enzymes, the most enigmatic is the Mg-protoporphyrin IX monomethyl ester cyclase (EC 18.104.22.168). This enzyme requires components found associated with the plastid membrane and the plastid soluble fraction. One of the components, XanL, is found associated with the membrane and another protein, Ycf54, has recently been identified based upon association with XanL. This study describes a deeper analysis of the role of Ycf54 in the enzyme and the localization of the protein in barley plastids. The results clearly demonstrate a strong association of Ycf54 with XanL, absence of Ycf54 from soluble fractions necessary for activity and more evidence for a third membrane localized component of the enzyme.